unity c#获取字符串所占字节

private void SetLimit (string str)
        {
            char [] chars = str.ToCharArray ();
            int num = 0;
            if ( str != null )
            {
                for ( int i = 0 ; i < chars.Length ; i++ )
                {
                    if ( isChinese (chars [i]) )
                    {
                        num += 2;
                    }
                    else
                    {
                        num++;
                    }
                }
            }
            if ( num < 20 )
            {
                _inputGuildName.characterLimit = 20;
            }
            else
            {
                _inputGuildName.characterLimit = str.Length;
            }
        }

        private bool isChinese (char v)
        {
            return v >= 0x4E00 && v <= 0x9FA5;
        }